Optical head having two semiconductor lasers of different wavelength, an objective lens focusing laser beams on different thickness substrates, and an annular phase shifter decreasing focused laser beam spot aberration

ABSTRACT

An objective lens and an optical head including the objective lens for reading information contained on substrates having different thicknesses using laser beams having different wavelengths. The objective lens includes an annular phase shifter for decreasing an aberration of a focused spot of each of the laser beams. The annular phase shifter can be optimally combined with the objective lens having inner and outer regions each having a different substrate thicknesses.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an optical disk apparatus foroptically reading information from an optical storage medium. Moreparticularly the present invention relates to an optical head forreading signals from optical disks having different substratethicknesses by using light sources having different optical wavelengths,and an objective lens for use in such optical head.

[0002] Optical disks have recently been making remarkable advances aslarge capacity-removable-information storage media. Accordingly,writing-reading methods, storage densities and disk sizes have taken ongreat diversity. Thus, it is becoming very difficult to ensurecompatibility among the different systems. Among other things, CDs(Compact Disc(s)) are presently most popular, and CD-Rs (CompactDisk-Recordable(s)) which are recordable CDs having readingcompatibility with CDs are becoming equally as popular. It is desiredthat development of new optical disks meet the important demand forcompatibility with such CDs and CD-Rs.

[0003] Recently DVDs (Digital Video Disk(s)), the next generation ofhigh density ROM following CDs and CD-Rs, have been introduced to themarket. To increase the storage density of a DVD, the numerical aperture(NA) of an objective lens is increased from 0.45 for the conventionalCDs up to 0.6. Letting λ be the wavelength of a laser source to be used,the size of a focused spot on an optical disk is proportional to λ/NA,so that as the wavelength is made shorter and the NA larger, the size ofthe beam spot can be made smaller. If the size of the beam spot issmall, it is possible to read high-density information pits with goodquality, so that the storage density of the optical disk can beincreased.

[0004] In light of the above, the wavelength of a semiconductor laserused for DVDs is 650 nm instead of 780 nm for CDs. However, since anincrease in the NA sharply increases coma which occurs when a disktilts, and rather degrades the beam spot, it is impossible toexcessively increase the NA. For this reason, DVDs have a substratethickness of 0.6 mm thinner than 1.2 mm of CDs so that the NA can beincreased and the accompanying coma due to a disk tilt can be reduced.However, since the substrate thickness of DVDs differs from that of CDs,if a CD is read with a DVD-dedicated objective lens, a sphericalaberration will occur and the beam spot will defocus. This occursbecause objective lenses for optical disks are respectively intended forparticular substrate thicknesses and are beforehand designed to havespherical aberrations which compensate for the particular substratethicknesses.

[0005] Conventional apparatus for solving the above problem is describedin, for example, Optical Review, Vol. 1, No. 1 (1994) pp. 27-29). Inthis conventional apparatus, a hologram is formed on the surface of anobjective lens for 0.6 mm disks, and a CD is read with diffracted light,while a DVD is read with transmitted light. The pattern of the hologramis beforehand designed so as to compensate for spherical aberrationwhich occurs during CD-read. However, in this conventional apparatus,since the hologram is used, a beam spot for DVDs is produced even duringa CD-read operation, whereas a beam spot for CDs is produced even duringa DVD-read operation. In addition, a beam reflected from a disk is againdiffracted. This leads to the disadvantage of unavoidable loss of lightpower.

[0006] A second conventional apparatus is described in MitsubishiElectric Co. Ltd. News Release, No. 9507 (Jun. 21, 1995). In the secondconventional apparatus, both an objective lens for 0.6 mm disks and anobjective lens for 1.2 mm disks are provided on an optical head, and thetwo lenses are switched when needed by a movable actuator. However, inthis example, since the two lenses are switched when needed, there areproblems such as an increase in cost due to the use of two lenses, thereproducibility of the positions of the lenses, and the degradation ofresponse characteristics due to a large and heavy actuator.

[0007] A third conventional apparatus is described in NikkeiElectronics, Jan. 29, 1996 (No. 654), pp. 15-16. In the thirdconventional apparatus an aperture limitation using a liquid crystal isprovided, and during a CD-read operation, the NA is reduced to 0.35 soas to reduce aberration. Since a semiconductor laser of wavelength 635nm is used for both CDs and DVDs, the NA for CDs can be reduced to someextent. There is, however, a disadvantage in that this method cannot beused for reading CD-Rs whose reflectance becomes quite low for a beam ofwavelength shorter than 780 nm.

[0008] A fourth conventional apparatus is described in Japanese PatentApplication No. 342203/1995. The fourth conventional apparatus providesan objective lens in which the inner and outer regions are givendifferent optimized substrate thicknesses, so as to realizecompatibility between both DVDs and CDs at a wavelength of 650 nm.However, if a CD is to be read at a wavelength of 780 nm, this boundaryNA needs to be made at least 0.45 or more, but this case leads to thedisadvantage that the aberration for DVD-read becomes extremely large.

SUMMARY OF THE INVENTION

[0009] An object of the present invention is to provide an optical diskapparatus having an optical head for reading signals from optical diskshaving different substrate thickness by using light sources havingdifferent optical wavelengths.

[0010] Another object of the present invention is to provide anobjective lens for use in an optical head for reading signals fromoptical disks having different substrate thicknesses by using lightsources having different optical wavelengths.

[0011] Yet another object of the present invention is to provides anoptical head for reading a CD having a substrate thickness of 1.2 mm byusing a beam of wavelength 780 nm as well as a DVD having a substratethickness of 0.6 mm by using a beam wavelength of 780 nm, without lossof light power, at low cost and with high precision.

[0012] The present invention provides an objective lens for focusing twolaser beams having different wavelengths on an optical disks havingdifferent thicknesses. Integrally added to objective lens is an annularphase shifter for decreasing aberrations of focused spots of therespective wavelengths.

[0013] A second embodiment of the present invention provides anobjective lens having different substrate thicknesses in the inner andouter regions of the objective lens for focusing a laser beam withoutaberration. Integrally added to the objective lens is an annular phaseshifter for decreasing aberrations of focused spots of laser beams ofdifferent wavelengths.

[0014] A third embodiment of the present invention provides an opticalhead which includes at least two semiconductor lasers having differentwavelengths, a diverging apparatus for diverging a beam reflected froman optical disk from an optical path which extends from thesemiconductor lasers to the optical disk, and a detector for detecting afocused spot position control signal and a data signal from thereflected beam diverged by the diverging apparatus. The optical headfurther includes an objective lens for focusing beams having therespective wavelengths on optical disks having different substratethicknesses.

[0015] A fourth embodiment of the present invention provides an opticalhead which includes at least two semiconductor lasers having differentwavelengths, an objective lens for focusing beams having the respectivewavelengths on optical disks having different substrate thicknesses, adiverging apparatus for diverging a beam reflected from an optical diskfrom an optical path which extends from the semiconductor lasers to theoptical disk, and a detector for detecting a focused spot positioncontrol signal and a data signal from the reflected beam diverged by thediverging apparatus. The optical head further includes an annular phaseshifter for decreasing aberrations of focused spots having therespective wavelengths.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention will be more apparent from the followingdetailed description, when taken in conjunction with the accompanyingdrawings, in which:

[0017]FIG. 1 is a diagram illustrating an objective lens according tothe present invention;.

[0018]FIG. 2 is a diagram illustrating a wavefront shape of a sphericalaberration;

[0019]FIG. 3 is a diagram illustrating a wave aberration shape obtainedfrom an annular phase shifter;

[0020]FIG. 4 is a diagram illustrating a wave aberration shape obtainedfrom an inverted annular phase shifter;

[0021]FIG. 5 is a table indicating phase shift for a CD under conditionswhich do not affect a DVD;

[0022]FIG. 6 is a graph illustrating spot performance for a CD-readoperation when a dual and a phase shifter are combined;

[0023]FIG. 7 is a graph illustrating a RMS wavefront aberrationoccurring during a DVD-read operation;

[0024]FIG. 8 is a diagram illustrating a dual optimum substrate lenswith which an optimized inverted annular phase shifter is formedintegrally;

[0025]FIG. 9 is a graph illustrating a variation in spot performance fora CD-read operation due to a shift of a CD-read wavelength;

[0026]FIG. 10 is a graph illustrating a RMS wavefront aberration for awavelength shift during a DVD-read operation;

[0027]FIG. 11 is a graph illustrating wave-aberration shapes for aCD-read operation;

[0028]FIG. 12 is a graph illustrating wave-aberration shapes for aDVD-read operation;

[0029]FIGS. 13A and 13B are a graph and a table illustrating the resultof calculations on spot shapes;

[0030]FIG. 14 is a diagram illustrating an embodiment of an optical headof the present invention;

[0031]FIG. 15 is a diagram illustrating an embodiment of the presentinvention in which an objective lens and an annular phase shifter areintegrated in a hybrid form; and

[0032]FIG. 16 are tables of the specification and shape of a DVD lens.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] Embodiments of the present invention will be described below withreference to the accompanying drawings.

[0034]FIG. 1 is a diagram illustrating an objective lens according tothe present invention. The construction of an objective lens accordingto the present invention makes use of a DVD objective lens 1. Accordingto the present invention a doughnut-shaped annular phase shift region101 is added to the DVD objective lens 1. The annular phase shift region101 may be formed as a thin film, or the DVD objective lens can beworked directly into such a shape in advance. Since a normal DVD lens isdesigned to have no aberration for a substrate thickness of 0.6 mm, whena DVD is to be read with a laser beam of wavelength 650 nm, anaberration is made as small as possible by the phase shifter. On theother hand, when a CD having a substrate thickness of 1.2 mm is to beread with a laser beam of wavelength 780 nm, a spherical aberration dueto a substrate thickness of 0.6 mm is decreased.

[0035] The manner in which an aberration decreases will be qualitativelydescribed below. FIG. 2 is a diagram illustrating the wavefront shape ofa spherical aberration for an optimized focus position. In FIG. 2, thehorizontal axis represents the coordinates of the pupil radius of theobjective lens, while the vertical axis represents a wave aberration. Abeam spot to be used for reading a CD with a DVD objective lens has awavefront shape such as that approximately expressed by a quarticfunction, because of the difference in substrate thickness between theCD and the DVD. FIG. 3 is a diagram illustrating a wavefront shapeobtained from an annular phase shifter. It can be seen that the maximumvalue of the aberration is made smaller by the annular phase shifter.

[0036] The aberration of a DVD must not become large when it is read byusing the above-described DVD objective lens. One method to compensatefor this is to use the difference between a wavelength for a CD-readoperation and a wavelength for a DVD-read operation so that a phaseshift occurs only when a CD is being read and no phase shift occurs whena DVD is being read. For this purpose, letting λ1 be the wavelength fora CD-read operation, λ2 the wavelength for a DVD-read operation, and Φ aphase shift occurring during a CD-read operation, the following isprovided:

(n+Φ)λ1=mλ2 (n, m: integer)  Eq. 1

[0037] The integers m and n may be selected to satisfy the aboveequation. If there is no appropriate m or n, the manner of the phaseshift may also be altered as shown in FIG. 4. In this case, a wavefrontshape identical to that shown in FIG. 3 can be realized by applying aphase shift of −Φ to the region other than the annular phase shiftregion. Therefore, in this case the following is provided:

(n−Φ)λ1=mλ2 (n, m: integer)  Eq. 2

[0038] From this equation, for example, if λ1 is set to 780 nm and λ2650 nm, the phase shift Φ in each region is as shown in FIG. 5. If thephase shift is selected in this manner, it is possible to decrease thespherical aberration during a CD-read operation without at all affectingthe wavefront for a DVD-read operation. The term “inverted annular phaseshifter” used herein is a name which takes into account the case inwhich a phase shift is realized by a phase lag, such as when a filmhaving a larger refractive index than air is added. In a case where aphase shift can be realized by a phase lead, as by grinding a lens, theannular phase shift region may be directly formed by grinding. Sinceeither case is equivalent, both cases will be hereinafter referred to asthe inverted annular phase shifter.

[0039] The shape of the annular phase shifter and the optimization ofthe phase shift will be described below. The Strehl intensity which isthe main peak intensity of a beam spot having an aberration normalizedwith the main peak intensity of an aberration-free spot is available asan evaluation index of a beam spot. However, with such Strehl intensity,a difference in NA in the presence of an aperture limitation does notappear. For this reason, the ratio of the main peak intensity of a beamspot to a total light power incident on the pupil of an objective lensis adopted as a new evaluation index when an aperture limitation ispresent. For example, even for the same aperture diameter, suchevaluation index becomes larger as the NA becomes larger, the spotdiameter becomes smaller or the main peak intensity becomes larger.$\begin{matrix}\begin{matrix}{\frac{\begin{matrix}{{spot}\quad {peak}} \\{intensity}\end{matrix}}{\begin{matrix}{{{total}\quad {incident}}\quad} \\{{light}\quad {power}\quad {in}\quad {the}} \\{{objective}\quad {lens}\quad {pupil}}\end{matrix}} = \frac{{{\int_{0}^{2\pi}{\int_{0}^{R}{^{i\quad {\phi {({r,\theta})}}}r{r}{\theta}}}}}^{2}}{{\int_{0}^{2\pi}{\int_{0}^{R}{r{r}{\theta}}}}\quad}} \\{= \frac{{{\int_{0}^{2\pi}{\int_{0}^{R}{^{i\quad {\phi {({r,\theta})}}}r{r}{\theta}}}}}^{2}{{\int_{0}^{2\pi}{\int_{0}^{R}{r{r}{\theta}}}}}^{2}}{{{\int_{0}^{2\pi}{\int_{0}^{R}{r{r}{\theta}}}}\quad }^{2}{\int_{0}^{2\pi}{\int_{0}^{R}{r{r}{\theta}}}}}} \\{= {{I_{st}\frac{( {\pi \quad R^{2}} )^{2}}{\pi \quad R^{2}}} = {I_{st}\pi \quad R^{2}}}}\end{matrix} & {{Eq}.\quad 3}\end{matrix}$

[0040] It can be seen from Eq. 3 that the new evaluation index isproportional to the product of the Strehl intensity and the second powerof the aperture limitation radius R normalized with the radius of thefull-aperture of the objective lens. In the following, n denotes thevalue obtained by multiplying the Strehl intensity by the second powerof the normalized aperture limitation radius. In a normal CD pickup,since a wavelength of 780 nm is used and an objective lens NA is 0.45,if there is no aberration for a DVD objective lens NA of 0.6,η=1×(0.45/0.6)²=0.56, and η=0.45 at 0.8 which is the lower limit of theStrehl intensity based on Marechal's criterion. Regarding sphericalaberration due to substrate thickness error, a fourth-order sphericalaberration is given by: $\begin{matrix}{W_{40} = {\frac{d}{8}\frac{n^{2} - 1}{n^{3}}({NA})^{4}}} & {{Eq}.\quad 4}\end{matrix}$

[0041] and a sixth-order spherical aberration is given by:$\begin{matrix}{W_{60} = {\frac{d}{32}\frac{n^{4} + {2n^{2}} - 3}{n^{5}}({NA})^{6}}} & {{Eq}.\quad 5}\end{matrix}$

[0042] In these equations, n denotes a refractive index. From theseequations, an aberration, obtainable by adding an annular phase shifterwhich causes a phase lag of Φ between a radius R1 and a radius R2, isexpressed as follows: $\begin{matrix}{{W = \{ \begin{matrix}{{W_{60}\rho^{6}} + {W_{40}\rho^{4}} + {W_{20}\rho^{2}} + W_{00\quad}} & ( {{0 \leq \rho \leq R_{1}},{R_{2} \leq \rho}} ) \\{{{W_{60}\rho^{6}} + {W_{40}\rho^{4}} + {W_{20}\rho^{2}} + W_{00} + \varphi}\quad} & {( {R_{1} \leq \rho \leq R_{2}} )\quad}\end{matrix} }} & {{Eq}.\quad 6}\end{matrix}$

[0043] The Strehl intensity can be approximated as follows:$\begin{matrix}\begin{matrix}{I_{st} = {1 - ( {\frac{2\pi}{\lambda}W_{rms}} )^{2}}} \\{= {1 - \{ {\frac{2\pi}{\lambda}( {\overset{\_}{W^{2}} - ( \overset{\_}{W} )^{2}} )} \}}}\end{matrix} & {{Eq}.\quad 7}\end{matrix}$

[0044] Therefore, from this equation, R1, R2 and Φ for a maximum η aswell as the NA, W20 and W00 of the aperture limitation are obtained.Actually, numerical-formula processing software was used to analyticallyobtain W20 and W00 and to numerically obtain R1, R2 and Φ as well as theNA of the aperture limitation. As the result, it was found that when theinner and outer diameters of the annular phase shifter were NA0.20 andNA0.42 and the NA of the aperture limitation was 0.456 and the phaseshift was 0.265 λ (λ=780 nm), η=0.48 became a maximum and larger thanη=0.45 based on Maréchal's criterion. On the other hand, whenoptimization was performed with only the aperture limitation withoutusing the annular phase shifter, η=0.45 was a maximum at NA0.39. Inother words, this is equivalent to an improvement of from 0.61 to 0.86in the Strehl intensity in terms of NA0.45. For this phase shift, theaberration occurring during a DVD-read operation was 0.33 λ (λ=650 nm)in terms of RMS wavefront aberration. This is almost equivalent to theworking accuracy of lenses, and it can be considered that no problemoccurs in practice. If this optimized phase shift of 0.265 λ is comparedwith the previously described phase shift which does not affect aDVD-read operation, it can be seen that the closest phase shift is 0.333λ of the inverted annular phase shifter for m=2 and n=2 or of theannular phase shifter for m=4 and n=3. However, as m becomes larger, thestep of a film or lens which causes a phase shift becomes thicker andthe deviation of a phase shift when a wavelength shift occurs in asemiconductor laser becomes larger. Therefore, the inverted annularphase shifter is more preferably herein. When the shape of a phaseshifter which was fixed at this phase shift which does not affect aDVD-read operation was obtained, η=0.47 was a maximum at an innerdiameter of NA0.20, an outer diameter of NA0.44, and an aperturelimitation of NA0.48. This is almost equal to the above-describedoptimized phase shift.

[0045] The effects of the annular phase shifter applying to the DVDobjective is confirmed by ray tracing. FIG. 16 shows the specificationsand the shape of the example DVD lens, where R, k, A4, A6, A8 and A10are the paraxial curvature radius, conical constant, 4-th, 6-th, 8-thand 10-th order of aspherical coefficients, respectively. The surfaceshape is defined using these parameters and radial coordinate r by$\begin{matrix}{{z(r)} = {\frac{r^{2}}{R + \sqrt{{R^{2}( {K + 1} )}r^{2}}} + {A_{1}r^{4}} + {A_{2}r^{6}} + {A_{3}r^{8}} + {A_{4}r^{10}}}} & {{Eq}.\quad 8}\end{matrix}$

[0046] where the shape is assumed to be symmetrical to the axis. Whenthe collimated light of wavelength of 780 nm is focused through CDsubstrate of thickness 1.2 mm without the annular phase shifter, theroot mean square (RMS) wave front aberration of the spot was 0.1279λ(λ=780 nm) in the aperture of NA 0.45. By applying the annular phaseshifter of 0.3333 λ (λ=780 nm) to this lens, however, the aberration wasreduced to 0.07366 λ (λ=780 nm). On the other hand, when the collimatedlight of wavelength of 650 nm is focused through DVD substrate ofthickness 0.6 nm with the annular phase shifter, the RMS wave frontaberration of the spot was accordingly less than 0.001 λ (λ=650 nm) inthe aperture of NA 0.6.

[0047] The above description has been made on the assumption that theaperture limitation is employed, but this does not necessarily mean thatan actual aperture is needed. Actually, it can be considered that theabove-described process is almost equivalent to specifying an evaluationarea of a pupil when an optimized focus position is to be obtained byusing an RMS wavefront aberration as an evaluation function. If a focuserror is adjusted so that the RMS wavefront aberration becomes as smallas possible within the area of the aperture limitation, the aberrationof light outside the area of the aperture limitation naturally becomeslarger and the slope of the wavefront also becomes larger. For thisreason, the rays in such area cross a focal plane at a position greatlyoffset from a focus. Therefore, the presence of such rays is almostequivalent to the absence of the rays in terms of a focused spot.

[0048] If only the annular phase shifter is used in the above-describedmanner, spot performance will be improved but a Strehl intensity of 0.86for NA0.45 may not completely suffice when account is taken of adegradation of a spot due to misalignment of optical parts, disk tilt,focus error or the like. For this reason, in combination with the aboveconstruction, different substrate thicknesses to be optimized may beprovided inside and outside a lens. Such lens is hereinafter referred toas the dual optimum substrate lens (DOSL). This lens was invented by thepresent inventors as a method of realizing compatibility between bothDVDs and CDs at a wavelength of 650 nm. This method is disclosed inJapanese Patent Application No. 342203/1995. However, such lens has thedisadvantage that it is necessary to make this dual NA at least NAO.45or more for the purpose of reading CDs at a wavelength of 780 nm, and inthis case, the aberration for DVD-read becomes extremely large.

[0049] To solve such disadvantages, a phase shifter and the dual optimumsubstrate lens were combined to optimize the shape of the phase shifter,a phase shift, an inside-outside boundary radius and an inside substratethickness at the same time, and it has been found out that suchcombination has the effect of decreasing both aberrations which occurdue to the dual optimum substrate lens during a CD-read operation at awavelength of 780 and during a DVD-read operation at a wavelength of 650nm, thereby further improving the spot performance for a CD-readoperation. This combination will be described below.

[0050] The wave aberration due to the combination of the dual optimumsubstrate lens and the phase shift is expressed as follows:$\begin{matrix}{W = \{ \begin{matrix}{{{W_{601}\rho^{6}} + {W_{401}\rho^{4}} + {W_{201}\rho^{2}W_{001}}}\quad} & {( {0 \leq \rho \leq R_{1}} )\quad} \\{{W_{601}\rho^{6}} + {W_{401}\rho^{4}} + {W_{201}\rho^{2}W_{001}} + \varphi} & ( {R_{1} \leq \rho \leq R_{2}} ) \\{{W_{602}\rho^{6}} + {W_{402}\rho^{4}} + {W_{202}\rho^{2}W_{002}} + \varphi} & ( {R_{2} \leq \rho \leq R_{3}} ) \\{{{W_{602}\rho^{6}} + {W_{402}\rho^{4}} + {W_{202}\rho^{2}W_{002}}}\quad} & ( {R_{3} \leq \rho \leq R_{4}} )\end{matrix} } & {{Eq}.\quad 8}\end{matrix}$

[0051] In this equation, R1 denotes the inner diameter of the annularphase shifter, R2 the boundary radius, R3 the outer diameter of theannular phase shifter, and R4 the radius of the aperture limitation. Thedisk substrate thickness required to remove an aberration differsbetween inner and outer regions separated by the boundary radius, andthe inner region is 0.6 mm thick for a DVD-read operation, while theouter region is optimized to be between 0.6 mm thick and 1.2 mm thick.Accordingly, the discriminator “1” or “2” is affixed to each of theaberration coefficients W60 and W40 for spherical aberration for thepurpose of discrimination between the inner and outer regions. The focuserrors W201 and W202 are determined from a spherical aberration so as tominimize the RMS wavefront aberrations of the inner region and the outerregion, and the constant terms W001 and W002 are determined so that theaverage values of the wave aberration of the inner and outer regionsbecome the same, thereby optimizing the total RMS wavefront aberration.The differences between W201 and W202 and between W001 and W002 aredetermined by the difference between the inner and outer correspondingsubstrate thicknesses of the lens, and W201 and W001 were alsoanalytically obtained by numerical-formula processing software underconditions for minimizing the RMS wavefront aberration under theconditions of the phase shifter which were given W202 and WO02.

[0052] Further, regarding the given inner corresponding substratethickness and the boundary radius R2, conditions for maximizing η wereobtained by numerically changing R1, R3, R4 and the phase shift. Theresult is shown in FIG. 6. In FIG. 6, the horizontal axis represents theboundary radius of the dual optimum substrate lens and the vertical axisrepresents η, and the result of calculations under optimized conditionsfor different central substrate thicknesses is plotted. In the graph,dashed lines respectively indicate a CD having no aberration, a lowerlimit level equivalent to a Strehl intensity of 0.8, the above optimizedphase shifter, and a fixed phase shifter. These lines cannot be plottedwith points on the graph because no dual optimum substrate lens is used.RMS wavefront aberration occurring during a DVD-read operation at thattime is shown in FIG. 7.

[0053] As can be seen from a comparison of FIGS. 6 and 7, as the centralsubstrate thickness is made closer to 1.2 mm, the performance for CDsbecomes higher and the aberration for DVDs increases. Therefore, adecision as to which of these points should be selected as an optimumpoint depends on the distribution of various margins of the system. Itis considered, however, that it is almost possible to accept a CDperformance of n=0.526 (0.94 in terms of the Strehl intensity for CDs)and a DVD RMS wavefront aberration of 0.030 for a central substratethickness of 0.76 mm and a boundary radius of NA0.45. When the phaseshifter is not provided, the evaluation factor of a CD and DVDaberration are 0.414 and 0.031, respectively. Accordingly, theaberration of the light DDot for both a CD and a DVD are decreased.Furthermore, at this point, the maximum value of the CD performance andthe minimum value of the DVD aberration coincide with each other. Thephase shift of the annular phase shifter at this time is 0.2985 λ (λ=780nm), the inner diameter is NA0.2145, and the outer diameter is NA0.45which coincides with the boundary radius NA.

[0054]FIG. 8 is a diagram illustrating a dual optimum substrate lenswith which an inverted annular phase shifter is formed integrally. Sincethe inverted annular phase shifter is formed integrally with the lens,the region of the annular phase shifter is recessed. Although a step forthe dual optimum substrate lens is also formed on a disk-side surfacehaving a comparatively moderate curvature, this step may be provided ononly an image side.

[0055]FIG. 9 illustrates the value of the CD-read spot performance ηaffected by a shift of a CD-read wavelength.

[0056] Although the range of the horizontal axis is ±20 nm, thewavelength range in which the value of η shifts due to temperaturevariations or the like seems to be about ±10 nm. The degradation withinthe wavelength range is from η=0.53 to approximately η=0.52 at awavelength shift of −10 nm, and is almost negligible because of avariation of from 0.93 to about 0.92 in terms of the Strehl intensityfor NA0.45. In addition, FIG. 9 illustrates the values of thepreviously-described optimized annular phase shifter and the fixedannular phase shifter.

[0057]FIG. 10 illustrates a RMS wavefront aberration for a wavelengthshift during a DVD-read operation at a wavelength of 650 nm, and theaberration increases from 0.030 λup to 0.036 λat a wavelength shift of−10 nm when the dual optimum substrate lens and the optimized annularphase shifter are combined. It can be considered that this increase isalso within a fully allowable range. In addition, FIG. 10 illustratesthe aberrations of the previously-described optimized annular phaseshifter and the fixed annular phase shifter. Regarding the fixed annularphase shifter, since its phase shift is selected so that no aberrationoccurs for DVDs, the aberration is 0 at a wavelength shift of 0.Regarding the optimized annular phase shifter, since its phase shift isoffset from a phase shift which causes no aberration for DVDs, the waveaberration linearly varies toward a wavelength shift which correspondsto the phase shift. FIG. 11 is a graph illustrating wave-aberrationshapes for a CD-read operation at a wavelength of 780 nm. For each ofthe wave-aberration shapes, since a focus error is optimized within theNA range of an aperture limitation and the horizontal axis representsthe coordinates of the pupil radius over a full-aperture of NA0.6, theaberrations become extremely large in their peripheral portions. On theother hand, since the vertical axis represents the aberrations in afolded form within the range of ±0.5 λ, the peripheral portions seem tobe sharply vibrating. These aberrations are suppressed over an NA widerthan when a focus error is optimized with only the aperture limitation.Furthermore, the rise of the wavefront outside the range of the aperturelimitation NA is also sharp, and it is expected that the effect of theaperture limitation becomes more remarkable because of such a largeaberration. FIG. 12 illustrates wave aberrations for a DVD-readoperation at a wavelength of 650 nm. Since the wave aberration for onlythe aperture limitation and that for only the fixed phase shifter, bothof which are shown in FIG. 11, become completely zero in FIG. 12, FIG.12 only illustrates the case in which the dual optimum substrate lensand the optimized phase shifter are combined and the case of only theoptimized phase shifter. From the fact that the aberration does notbecome zero even in the outermost portion in which no aberration at alloccurs, it is seen that a small focus error occurs over the entirepupil. This is because the total RMS wavefront aberration becomes smallowing to the small focus error if the phase shift caused by the phaseshifter is regarded the aberration. In any case, the value of thevertical axis of the graph is considerably small and the distinctivenessof the wavefront shape is reduced to an actually negligible degree ofRMS wavefront aberration.

[0058]FIG. 13A illustrates the result of calculations on spot shapes. Inthe graph, the horizontal axis represents the spot size of an intensitywhich is exp (−2) times the peak intensity of a spot, while the verticalaxis represents the value of the intensity of a side-lobe normalizedwith the main peak intensity of the spot. Accordingly, since it isdesirable that both the spot and the side-lobe be small, it follows thata plotted point nearer to the bottom left of the graph corresponds to aspot of higher resolution. Assuming that the intensity distribution ofthe pupil of the objective lens is a symmetrical Gaussian distribution,the shown calculation result is that obtained when the ratio of a lensdiameter to the range of the intensity of exp (−2) times the intensityof the center of the Gaussian distribution in the pupil is 0.1 and theratio of the intensity of the peripheral portion of the lens to theintensity of the central portion thereof is 0.98.

[0059] In FIG. 13B, a white circle denotes a normal CD lens having noaberration, and as the position of a plotted point is nearer to thewhite circle, read-out performance becomes closer to that of the normalCD lens. Each black square denotes a normal DVD lens with only anaperture limitation. As such black squares, there are plotted threepoints which respectively correspond to the case in which the aperturelimitation is actually inserted, the case in which the aperturelimitation is omitted at that focus position, and the case in which theaperture limitation is omitted and the focus position is shifted so thatthe spot peak intensity becomes a maximum. Any of the cases is inferiorin spot resolution to the normal CD lens having no aberration. Eachblock triangle denotes the case in which only the optimized annularphase shifter is inserted, and there are similarly three plotted points.

[0060] Although the spot size is considerably improved as compared withthe case of the aperture limitation only, the side-lobe becomesconsiderably large if there is no aperture limitation. Each white squaredenotes the case in which the dual optimum substrate lens and theoptimized annular phase shifter are combined. Although there aresimilarly three plotted points, it is seen that these three plottedpoints are considerably close to one another. In other words, it is seenthat in this case it makes no matter whether there is an aperturelimitation or not, and since the aberration of a beam outside the rangeof a virtual aperture limitation sharply increases, the forming of aspot is not substantially affected.

[0061] In this case, the beam spot is slightly smaller in spot size andslightly larger in side-lobe than the normal CD lens having noaberration.

[0062] The reason why the value of η which is the evaluation index ofthe spot performance, is almost equal or slightly inferior to the normalCD lens having no aberration is presumed to be that the effect of theside-lobe which is not completely decreased is canceled by reducing thespot size. In addition, the result of calculations on a spot for aDVD-read operation is plotted with a white triangle and diamond at thebottom left. The diamond denotes a spot for reading a DVD withoutaberration, and the triangle denotes the case in which the optimizeddual optimum substrate lens and the optimized annular phase shifter arecombined. Spot shapes for DVDs are almost the same.

[0063]FIG. 14 illustrates an embodiment of an optical head.

[0064] Light from semiconductor lasers 41 and 42 of differentwavelengths are combined by dichromatic mirror 20 and formed intoparallel light by a collimator lens 5. The elliptical beam is formedinto a circular beam by beam forming prisms 61 and 62. If the efficiencyof the optical system is sufficiently high or the track pitch of a diskis wider than the gap between a main lobe of a beam spot and a firstdark line on the disk, the beam forming prisms can also beadvantageously omitted in terms of the number of component parts or forthe purpose of decreasing crosstalk between adjacent tracks. The beam istransmitted through a beam splitter 71 and reflected by an erect mirror8, and is then focused on an optical disk 10 by an objective lens 3according to the present invention. The objective lens 3 is provided ona two dimensional actuator 9. The optical disk 10 may be a CD or a DVD.The two-dimensional actuator 9 moves in the direction of a disk radiusin response to a tracking error signal and positions the beam spot on atrack, and also moves in the direction of the optical axis in responseto a focus error signal and positions a focus position on the disk.

[0065] The reflected beam again passes through the objective lens 3 andthe erect mirror 8, and is reflected by the beam splitter 71 andconducted toward a detecting optical system. The beam transmittedthrough a beam splitter 72 is formed into a focused beam by a focusinglens 111, and enters a beam splitter 73. The beam transmitted throughthe beam splitter 73 is transmitted through a cylindrical lens 12 and ismade incident on a four-split photodetector 13. A differential signalobtained from the sum signals of the diagonal components of this splitphotodetector is outputted from a differential amplifier 141 as a focuserror signal.

[0066] The beam reflected by the beam splitter 73 is also made incidenton a two-split photodetector 15, and a differential signal obtained fromthe outputs of the two-split photodetector 15 is outputted from adifferential amplifier 142 as a tracking error signal. The beamreflected by the beam splitter 72 is focused on a photodetector 16 by afocusing lens 112, and the signal photoelectrically converted by thephotodetector 16 is amplified by an amplifier 17 so that a data signalis obtained. The data signal may be detected from a sum signal of theoutputs from the detector for detecting a servo signal. In this case,the servo signal may be detected by band-limiting a signal detected upto a signal band, through a low-pass filter or the like. The servodetecting optical system is one example, and another system may also beused.

[0067] Although the above description has referred to the embodiment inwhich the annular phase shifter is formed integrally with the objectivelens, FIG. 15 shows another embodiment in which a DVD objective lens 18and an independent annular phase shifter 19 are integrated in a hybridform as an optical head which is incorporated in a two-dimensionalactuator. Since it is assumed that this embodiment is substituted foronly a portion corresponding to the optical system of FIG. 14 whichextends from the erect mirror to the disk, FIG. 15 shows only thecorresponding portion.

[0068] By using an annular phase shifter or by optimally combining theannular phase shifter and an objective lens having inner and outerregions each having a different substrate thickness which causes noaberration, it is possible to read a DVD having a substrate thickness of0.6 mm with a laser beam of wavelength 650 nm and a CD having asubstrate thickness of 1.2 mm with a laser beam of wavelength 780 nm byone lens without the need for an aperture limitation. Thus, using thepresent invention it is possible to provide a small-size inexpensiveoptical head.

[0069] While the present invention has been described in detail andpictorially in the accompanying drawings it is not limited to suchdetails since many changes and modifications recognizable to those ofordinary skill in the art may be made to the invention without departingfrom the spirit and the scope thereof.

We claim:
 1. An objective lens for focusing two laser beams havingdifferent wavelengths through substrates having different thicknessesfor the respective wavelengths, comprising: an annular phase shifter fordecreasing an aberration of a focused spot of each of the two laserbeams.
 2. An objective lens according to claim 1, wherein saidsubstrates include a substrate having a 1.2 mm thickness and a substratehaving 0.6 mm thickness.
 3. An objective lens according to claim 1,wherein said annular phase shifter is integrally formed in the objectivelens.
 4. An objective lens according to claim 1, wherein said laserbeams include a laser beam having a wavelength of 650 nm and a laserbeam having a wavelength of 780 nm.
 5. An objective lens for focusingtwo laser beams having different wavelengths through substrates havingdifferent thicknesses for the respective wavelengths, comprising: anannular phase shifter for decreasing an aberration of a focused spot ofeach of the two laser beams, Wherein the two laser beams are focusedwithout aberrations by an inner region and an outer region of saidobjective lens.
 6. An objective lens according to claim 5, wherein saidsubstrates include a substrate having a 1.2 mm thickness and a substratehaving 0.6 mm thickness.
 7. An objective lens according to claim 5,wherein said annular phase shifter is integrally formed in the objectivelens.
 8. An objective lens according to claim 5, wherein said laserbeams include a laser beam having a wavelength of 650 nm and a laserbeam having a wavelength of 780 nm.
 9. An optical head comprising: twosemiconductor lasers for generating two laser beams having differentwavelengths; an objective lens for focusing two laser beams havingdifferent wavelengths through substrates having different thicknessesfor the respective wavelengths, wherein said objective lens comprises:an annular phase shifter for decreasing an aberration of a focused spotof each of the two laser beams; a diverger which diverges a beamreflected from an optical disk on an optical path which extends fromsaid semiconductor lasers to the optical disk; and a detector whichdetects a focused spot position control signal and a data signal fromthe reflected beam diverged by said diverger.
 10. An optical headaccording to claim 9, wherein said substrates include a substrate havinga 1.2 mm thickness and a substrate having 0.6 mm thickness.
 11. Anoptical head according to claim 9, wherein said annular phase shifter isintegrally formed in the objective lens.
 12. An optical head accordingto claim 9, wherein said laser beams include a laser beam having awavelength of 650 nm and a laser beam having a wavelength of 780 nm. 13.An optical head comprising: two semiconductor lasers for generating twolaser beams having different wavelengths; an objective lens for focusingtwo laser beams having different wavelengths through substrates havingdifferent thicknesses for the respective wavelengths, wherein saidobjective lens comprises: an annular phase shifter for decreasing anaberration of a focused spot of each of the two laser beams; wherein thetwo laser beams are focused without aberrations by an inner region andan outer region of said objective lens; a diverger which diverges a beamreflected from an optical disk on an optical path which extends fromsaid semiconductor lasers to the optical disk; and a detector whichdetects a focused spot position control signal and a data signal fromthe reflected beam diverged by said diverger.
 14. An optical headaccording to claim 13, wherein said substrates include a substratehaving a 1.2 mm thickness and a substrate having 0.6 mm thickness. 15.An optical head according to claim 13, wherein said annular phaseshifter is integrally formed in the objective lens.
 16. An optical headaccording to claim 13, wherein said laser beams include a laser beamhaving a wavelength of 650 nm and a laser beam having a wavelength of780 nm.
 17. An optical head comprising: at least two semiconductorlasers having different wavelengths; an objective lens for focusingbeams having the respective wavelengths on optical disks havingdifferent substrate thicknesses; an annular phase shifter for decreasingboth aberrations of focused spots having the respective wavelengths;diverging means for diverging a beam reflected from an optical disk froman optical path which extends from said semiconductor lasers to theoptical disk; and detecting means for detecting a focused spot positioncontrol signal and a data signal from the reflected beam diverged bysaid diverging means.
 18. An optical head according to claim 17, whereinsaid substrates include a substrate having a 1.2 mm thickness and asubstrate having 0.6 mm thickness.
 19. An optical head according toclaim 17, wherein said annular phase shifter is integrally formed in theobjective lens.
 20. An optical head according to claim 17, wherein saidlaser beams include a laser beam having a wavelength of 650 nm and alaser beam having a wavelength of 780 nm.
 21. An optical headcomprising: at least two lasers having different wavelengths; anobjective lens for focusing beams having the respective wavelengths onoptical disks having different substrate thicknesses, said objectivelens having different substrate thicknesses for focusing the beamswithout aberrations in its inner and outer regions; an annular phaseshifter for decreasing both aberrations of focused spots having therespective wavelengths; diverging means for diverging a beam reflectedfrom an optical disk from an optical path which extends from saidsemiconductor lasers to the optical disk; and detecting means fordetecting a focused spot position control signal and a data signal fromthe reflected beam diverged by said diverging means.
 22. An optical headaccording to claim 21, wherein said substrates include a substratehaving a 1.2 mm thickness and a substrate having 0.6 mm thickness. 23.An optical head according to claim 21, wherein said annular phaseshifter is integrally formed in the objective lens.
 24. An optical headaccording to claim 21, wherein said laser beams include a laser beamhaving a wavelength of 650 nm and a laser beam having a wavelength of780 nm.